1. Field of the Invention
This invention relates to a plasma processing apparatus applied in the field for manufacturing, e.g., a semiconductor device and a plasma processing method using such an apparatus, and more particularly to an apparatus capable of controlling production ratio between ions and radicals in so called high density plasma, and a method in which such an apparatus is used to carry out, with high accuracy, plasma processing represented by dry etching.
2. Description of the Related Art
With improvements in the degree of integration of semiconductor devices as represented by VLSI, and further ULSI, reduction of the minimum processing dimension thereof is being rapidly developed. For example, in 84M DRAM which will be mass-produced in the near future, design rule of 0.35 .mu.m is adopted. Moreover, when viewed from the level of researches, aiming at manufacturing of the next generation and the further next generation semiconductor devices, studies on miniaturization processing based design rule of 0.25.about.0.1 .mu.m are being developed.
Dry etching which discharges and dissociates etching gas within a high vacuum chamber to etch samples by making use of ions or radials in produced plasma is a technology which has greatly contributed to development of such miniaturization technology. In the present circumstances, ECR (Electron Cyclotron Resonance) system and magnetron RIE system are the main current of plasma production.
The ECR system allows electrons to undergo spiral (screw) movement under the electron cyclotron resonance condition based on interaction between magnetic field of 8.75.times.10.sup.-2 T (875G) and microwave (2.45 GHz) to thereby increase the probability of collision between electrons and gas molecules, thereby permitting attainment of ion density of the order of 10.sup.11 /cm.sup.3 even under the condition of low gas pressure of 0.01.about.0.33 Pa (10.sup.-3 .about.10.sup.-5 Torr). This system also has the great merit that wafer bias is used in combination so that plasma density and ion energy can be independently controlled.
On the other hand, the magnetron RIE system applies transverse magnetic field to electric field to thereby confine electrons within plasma to allow them to carry out cycloid movement in the vicinity of the wafer to elongate travel and life time to thereby improve ionization rate even under the condition of low gas pressure. In accordance with this system, it is possible to attain ion density of the order of 10.sup.10 /cm.sup.3.
However, when attempt is made to form, with good accuracy, fine pattern of the minimum processing dimension 0.25 .mu.m required in 256M DRAM or dimension narrower than that, it is becoming clear that there is limitation in the case of existing plasma.
For example, in both the ECR system and the magnetron RIE system described above, strong magnetic field is used in order to improve plasma density. However, in recent processes in which large diameter wafer having diameter ranging up to 8 inches, it is difficult to allow such magnetic field to be uniform over the entire surface of the wafer. As a result, plasma density becomes uneven and breakdown of gate insulating film is apt to occur. Moreover, there occurs the problem that electrons stored in the wafer affects the incident direction of ions resulting from the fact that capture rates with respect to magnetic field of electrons and ions under strong magnetic field are different, giving rise to extraordinary etching shape.
This problem appears in a form of occurrence of side etch into W film 5 in etching of Al/W system stacked (laminated) wiring film as shown in FIG. 1, for example, in practice. This figure shows the state where as the result of the fact that Al/W system stacked wiring film is etched on SiO.sub.X interlayer insulating film 1, anisotropic shapes are obtained with respect to Ti film 2, TiN film 3 (which are referred to as barrier metal 4), Al-1% Si film 6 and TiON reflection protective film 7, but side etch takes place with respect to W film 5.
Under such fine design rule, essential anisotropic etching based on clean process which does not rely upon side wall protection is desired, but discharge under low gas pressure condition is required for this purpose. However, density of chemical species in plasma contributing to etching is lowered under low gas pressure condition as a matter of course, giving rise to the problems of low etching speed (etch rate) and low throughput.
While realization of lower gas pressure, higher plasma density, and lower magnetic field in the vicinity of wafer is required compared to the prior art, several kinds of new density plasma sources have been recently proposed in succession. As one of expected high density plasma sources, there is high density plasma source using helicon wave plasma described in, e.g., the Japanese Laid Open Patent No. 3-68773 (No. 68773/1991). In accordance with the production mechanism thereof, magnetic field is applied to cylindrical chamber to further apply high frequency to loop antenna wound on the chamber to produce helicon wave within the chamber to accelerate electrons by energy transportation from the helicon wave to electrons through the process of Landau attenuation to allow the accelerated electrons to impinge on gas molecules thus to obtain high ionization rate. In the case of helicon wave plasma, ion density of approximately 10.sup.11 .about.10.sup.13 /cm.sup.3 can be attained.
Since the above-described helicon wave plasma does not require strong magnetic field as used in the conventional ECR system or magnetron system for its production, magnetic field in the vicinity of the wafer can be reduced to much degree, or can become equal to substantially zero. Accordingly, unevenness of plasma density, vibration of ion and oblique incidence of ion, etc. due to the influence of magnetic field can be all reduced to much degree, and breakdown of gate insulating film or occurrence of extraordinary etching shape can be suppressed. In addition, since expensive magnetic coil and/or large-sized microwave source are not required, and relatively low frequency can be adopted as a frequency of high frequency power source used, the system can become compact, simplified and inexpensive. This is extremely great merit when the fact that the configuration of the device manufacturing system will be directed to multi-chamber configuration is taken into consideration.
However, since electron temperature is extremely high in plasma of low pressure and high density like helicon wave plasma, dissociation of gas molecules is excessively developed. As a result, the problem that ions are excessively produced, whereas quantity of radicals produced is insufficient would take place. Generally, high etch rate in dry etching is attained by so called ion assist mechanism to hasten chemical reaction by radicals adsorbed on the surface of sample by physical energy given in the form of ion impact. Accordingly, in the system where radicals are insufficient, adsorption of radicals and ion irradiation cannot be smoothly repeated. As a result, etch rate is disadvantageously lowered.